Elisabeth of Bourbon was the first wife of Philip IV of Spain. In 1644 she died pregnant of her eighth child. She and Philip had just one daughter who reached adulthood: Maria Theresa, the future wife of Louis XIV of France. Their only son, Baltasar Carlos, died probably from appendicitis in 1646. After her death, Philip married Mariana of Austria, who gave him two surviving children: Margaret (who married the Emperor Leopold I) and the deformed Charles II.
But WI Elisabeth had not died in 1644 but had lived maybe 20 years more? Assuming that the baby she would give birth dies in infancy, as the majority of the other children she had, and Baltasar still dies from appendicitis, then the only heir of Philip IV would be Maria Theresa. I doubt she would be married to Louis XIV in this situation, because Philip would do everything possible to not create a personal union of France and Spain. So, who could be the possible husband of Maria Theresa? Who could Louis XIV marry? And how would this situation change the relation between both countries?
But WI Elisabeth had not died in 1644 but had lived maybe 20 years more? Assuming that the baby she would give birth dies in infancy, as the majority of the other children she had, and Baltasar still dies from appendicitis, then the only heir of Philip IV would be Maria Theresa. I doubt she would be married to Louis XIV in this situation, because Philip would do everything possible to not create a personal union of France and Spain. So, who could be the possible husband of Maria Theresa? Who could Louis XIV marry? And how would this situation change the relation between both countries?